The present invention relates to a bolus intended to receive irradiation during radiotherapy. It has a particularly useful but non-limitative application in breast radiotherapy.
By bolus is meant a material, if possible tissue-equivalent, placed in the contact with an irradiated region in order to correct surface irregularities or in order to give to the depth dose distribution a shape appropriate for the anatomical structures to be protected or irradiated.
In general, radiotherapy is becoming unavoidable in the treatment of breast cancers, regardless of the stage of development of the tumor and the therapeutic protocol; it limits the risk of local recurrence by 70%. Thus, radiotherapy is indispensable in radical surgery where a mastectomy combined with axillary curage is followed by irradiation of the thoracic wall and the drainage lymph node areas. It is becoming essential with the development of conservative treatments, surgery being limited to a tumorectomy and axillary curage. In other words, radiotherapy is indispensable in all forms of breast cancer in particular, usually combined with surgery and sometimes with chemotherapy.
In France, 50% of radiotherapy treatments after mastectomy use electron beam irradiation.
Treatment consists of irradiating a target volume which can be:                the breast or the thoracic wall,        the axillary region,        the supraclavicular region,        the internal mammary chain.        
The problem associated with irradiation is therefore the presence of healthy organs in close proximity to the target to be treated. Irradiation of these healthy organs must be avoided, which can for example comprise:                the lungs,        the heart,        the brachial plexus,        the cervical spinal cord,        the larynx, or        the thyroid.        
FIG. 1 according to the prior art shows a curve representing the energy efficiency of an electron beam irradiation as a function of the depth (Z) in the body. Physical phenomena mean that the efficiency does not follow a linear decrease from the skin to the inside of the body. Instead the efficiency has a dose maximum at a certain depth relative to the skin and then rapidly decreases exponentially. In most cases, it is necessary to shift this maximum so that the latter is situated exactly on the target while providing maximum protection to the healthy organs. In FIG. 1 for example the dose maximum is at a depth of 20 mm. In order to carry out a shift, current practice consists of placing a bolus on the zone to be treated so as to artificially increase the thickness of the skin and thus reduce the depth of penetration of the irradiation beam. The bolus most commonly used is made of silicone sold in sections ranging from 5 to 10 mm thick. This silicone bolus is difficult to handle and remains expensive. FIG. 2 according to the prior art shows such a bolus placed on a body the surface of which has an irregular shape. Because of the relative rigidity of silicone, gaps 3, 4 and 5 form between the body 2 and the bolus 1. Moreover, it is clearly evident that the penetration of the electron beam is not homogeneous. The disadvantages of a silicone bolus are therefore essentially:                difficulty in handling,        a density other than 1, this silicone bolus is not tissue-equivalent,        high cost, and        available in sections of a fixed thickness, which does not allow it to fit properly to the irregular surface of a human body: this leads to inhomogeneity in the dose distribution in the target volume.        
Document U.S. Pat. No. 6,231,858 is known which describes a bolus for radiotherapy constituted by an aqueous gel. This gel is prepared from a mixture of natural organic polymers and water. This bolus is inexpensive and can be adapted so as to correct the dose distribution. However, to produce this bolus it is necessary to heat the water to a temperature between 70 and 100° C. before incorporating a natural organic polymer into it. This high-temperature preparation method is very restrictive and requires permanent and rigorous monitoring in order to avoid in particular the presence of air bubbles.
The article “Utilization of custom electron bolus in head and neck radiotherapy” by Kudchadker et al. (Journal of Applied Clinical Medical Physics, pp. 322-333, Vol. 4, number 4, Fall 2003) describes a conformational wax bolus for radiotherapy treatment of the head and neck tumors. Two boluses were designed from data generated by a TPS (Treatment Planning System). The boluses were machined from modelling wax with an approximate density of 0.92 gm/cm3 (p. 324, C).
The article by Perkins et al. “A custom three dimensional electron bolus technique for optimization of post-mastectomy irradiation” Int. J. Radiat. Oncol. Biol. Phys. 51, 1142-1151, describes the use of a compensating wax bolus in the radiotherapy of breast tumors. The authors describe the use of a wax bolus 3 to 4 cm thick and the application of 16 MeV of energy.
Moreover, in fields far removed and independent of radiotherapy, polyurethane-based materials are known for various applications.
Document FR 2 682 289 describes a heat- or cold-energy accumulator constituted by a polyurethane or silicone gel in a very flexible casing (p. 1., I. 31). This casing is necessary for the accumulator because one of the properties of the gel in general is that it is sticky (p 1., I. 31-33) and the cross-linking of the initial liquid components of the gel is easier in a casing.
Document U.S. Pat. No. 6,191,216 describes hydrophilic and self-adhesive polyurethane gel substances.
Document U.S. Pat. No. 5,120,816 describes polyurethane resins characterized by increased strength. The applications of these resins are in particular the manufacture of intravenous catheters (col. 1, I 9-53). U.S. Pat. No. 5,120,816 also describes, among other objects manufactured with such a resin, boluses for animals (col. 14, I. 9). The description (p. 10, I. 30-35) refers to boluses for animals such as cattle or sheep where the property of the increased strength of the polyurethane resin is important. Thus “bolus” is a medical term also denoting an intravenous injection of a significant dose of a therapeutic agent. The term “bolus” in document U.S. Pat. No. 5,120,816 is therefore different from the bolus as treated in the present invention, which is a tissue-equivalent material to even out a dose of irradiation.